Tin oxide coating



June 7, 1966 P. J. cLoUGH ETAL 3,255,035

TIN OXIDE COATING Filed Nov. 15, 1961 Fig. l

Fig. 2

United States Patent O 3,255,035 TIN OXIDE CATING Philip J. Clough, Reading, Paul L. Raymond, Beverly, and Robert W. Steeves, Nahant, Mass., assignors, by mesne assignments, to National Research Corporation, Cambridge, Mass., a corporation of Massachusetts Filed Nov. 15, 1961, Ser. No. 152,550 5 Claims. (Cl. 117-62) The present invention relates to coating and more particularly to a method of coating nonmetallic substrates with tin metal.

In the past considerable effort and attention has been directed to the vacuum vapor deposition of aluminum metal on nonmetallic substrates to produce bright specular metallic coatings for use is products requiring a high quality specular metallic appearance. The use of tin metal for providing such coatings has not 'been desirable since the tin coatings produced by thermal evaporation in a vacuum chamber were of poor quality. Heretofore tin coatings produced by vacuum deposition methods resulted in coatings which were, for example, milky white, hazy, or brown and non specular in appearance.

Accordingly, it is a principal object of the present invention to provide a method of depositing tin coatings of high specular metallic appearance on nonmetallic substrates by thermal evaporation in a vacuum chamber.

Another object of the present invention is to provide a method of producing coated transparent nonmetallic sheets, ribbons, and threads having a specular metallic appearance and which are highly resistant to dyeing solutions particularly alkaline dye solutions.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The present invention accordingly comprises the product possessing the features, properties, and the relation of components and the process involving the several steps and the relation and the order of one or more of such steps with respect to each of the others which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims. y

For a fuller understanding of the nature and objects of the invention, reference should fbe had to the following detailed description taken in connection with the accompanying drawings wherein:

FIG. l is an enlarged, diagrammatic, schematic View of one embodiment of the invention.

FIG. 2 is an enlarged schematic, cross-section of another embodiment of the invention.

'Ille method of depositing bright, specular coatings of tin on a non-metallic flexible substrate by thermal evaporation in a vacuum chamber, in accordance with the present invention, consists in sufficiently evacuating the chamber to provide a residual gas pressure below about 100 microns Hg abs., heating tin metal in a container to a temperature in excess of about 1400 C. to provide an evaporation rate of tin in excess of about 2 grams per minute per square inch of molten tin surface, positioning the substrate in the path of the tin vapors and moving the substrate across the tin vapor path. In this manner Patented June 7, 1966 ICC Thus, maintaining a residual gas pressure in the charnber of below about microns Hg abs. and heating the tin metal to a temperature in excess of about 1400 C. to provide an evaporation rate of tin in excess of about 2 grams per minute per square inch of molten tin surface comprise important features of practicing the present invention.

While the tin is initially deposited it is strongly adherent; however, its resistance to abrasion is Very slight and the tin coating can be easily smudged or scratched. Accordingly the invention also comprises treatment of the tin coating to improve its resistance to abrasion. The tin film is preferably oxidized immediately to provide at least a surface lm of tin oxide.

The hardening or oxidizing treatment, in accordance with the present invention, comprises subjecting the tin coating to hot humid or moist air for a period of about 4 minutes or to condensing steam for a period of about 20 seconds. Hot humid air or condensing steam is of particular advantage where the substrate consists of paper and where it is desired to condition the paper by introducing a predetermined amount of moisture. Thus the hardening process of the present invention can serve a dual purpose. Also it has been determined that the hardening process can be accelerated by subjecting the tin coating to dry steam for a period of about 10 seconds.

In a preferred embodiment of the invention for coating flexible transparent plastic sheets, the tin source temperature is such that about 6 grams of tin are evaporated per minute per square inc h of molten tin surface in the source. This high evaporation rate gives a shiny, dense, specularly reflecting tin film. The vacuum in the chamber is preferably maintained at a pressure on the order of 10 to 100 microns Hg abs. which provides for economical coating operations, particularly with semicontinuous coaters. The thickness of the tin deposited is preferably between about l and 5 millionths of an inch. This corresponds approximately to a measured resistivity of between 5 and l ohm per square. Coatings much in excess of 5 millionths of an inch do not add to the appearance of the product and are more expensive. Coatings much less than 1 millionth of an inch are, for most purposes, too transparent. The lrate of travel of the substrate through the tin vapors or the time of exposure of the substrate can be varried in relation to the rate of evaporation in order to achieve the desired coating thickness. The tin coating deposited is then treated as described above to improve its resistance to abrasion.

Where a laminated product is desired., the tin-coated plastic sheet is then laminated to another transparent plastic sheet which can be of the type normally used for making such plastic-metallic threads as fully described in the Prindle and Lacy Patent 2,714,569. This lamination is preferably achieved 'by use of suitable thermoplastic adhesives which may include dyes or other materials for producing special optical effects.

Referring now to the drawings -there are illustrated several preferred embodiments of the invention. In FIG- URE 1 the base sheet is indicated at 10 as carrying a thin layer of tin 12. Adjacent to the tin layer there is provided an adhesive layer 14 for securing the second sheet 16 to the base sheet 10. The adhesive layer 14 can contain a dye or ink if desired.

In another embodiment of the invention shown in FIGURE 2 a third layer of transparent plastic sheet 18 is laminated to the previously formed sandwich comprising sheets 1t) and 16. As illustrated this is laminated to layer 10 by means of adhesive layer 20 which can, if desired, contain suitable inks, dyes and the like. To produce patterns, layer 10 or 18 can be printed with stripes or the like prior to forming the final lamination'.

In the past, considerable commercial success has been attained for textile threads made in accordance with the techniques described in Prindle and Lacy Patent 2,714,569. Such products have received wide use in automobile fabric and other textile applications, but their utility in the iield of certain other textile applications has been limited by the fact that the aluminum coating utilized in such products is attacked by alkaline dyeing solution so as to partially destroy the laminate and also affect the optical properties of the thread.

The tin coated threads, prepared in accordance With the present invention, overcome this disadvantage of aluminum coated products. The tin coated products are highly resistant to alkaline and other dyeing solutions. Accordin-gly, the present invention provides an improved laminated thread having excellent resistance to alkaline dyes while still retaining brightness and specular reilectance.

The invention will now be described by way of nonlimiting examples thereof;

Example I In this example of the invention, tin-coated polyethylene terephthalate was prepared by heating a crucible containing tin to a temperature on the order of 1500D C. (as measured by an optical pyrometer with correction for transmission loss through the window of the vacuum coating chamber). The pressure in the vacuum coating chamber was about 60 microns. Hg abs. The substrate to be coated (polyethylene terephthalate) was moved over the source of tin vapors at a speed of up to about 1000 feet per minute. The source was an elongated boat extending across the substrate and conning a long molten pool of tin having a dimension of about 3A of an inch in the direction of motion of the substrate. This gave a lcalculated coating thickness of approximately 2 millionths of an inch which had a measured resistivity of about 2 to 3 ohms per square. The tin coating was bright and shiny, had a highly specular metallic appearance and adhered strongly to the polyethylene terephthalate sheet. The tin coated sheet was treated in warm moist air for about 4 minutes to oxidize the surface of the tin coating- This sheet was then laminated to another similar sheet and slit to a thread which could be immersed in an alkaline dye bath with no deleterious effect.

Example 2 In this example of the invention a sheet of polyvinyl fluoride was coated with tin. In this form of the invention a 1/2 mil thick polyvinyl fluoride sheet was cleaned by rinsing with isopropyl alcohol, and then subjecting it to an A C. glow discharge of 10,000 volts for l0 minutes in a'vacuum chamber at a pressure of about 200 microns. A crucible containing tin Was then heated to a temperature of about 1500 to 1600D C. (as measured by an optical pyrometer with correction for transmission loss through the window of the vacuum coating chamber). The pressure in the vacuum coating chamber was reduced to about .1 micron Hg abs. The polyvinyl fluoride sheet Was exposed to the tin vapors for about 1/2 a second and then removed. This gave a coating of about 1 to 2 millionths of an inch thick which had a measured resistivity of about 2 to 5 ohms per square. The tin coating was bright and shiny, had a highly specular metallic appearance and was strongly adherent to the polyvinyl fluoride sheet and could not be removed by the Scotch tape test. In this test, pressure-senstive Scotch cellophane tape was pressed on the metallized surface and then rapidly removed.

Example 3 substrate was moved over the source of tin vapors-at a speed of about 400 feet per minute. The tin coating produced while being suticiently thick and adherent was hazy, bluish and did not have a specular metallic appearance.

Example 4 In this example the tin coated polyethylene terephthalate was obtained by heating the Crucible containing tin to a temperature on the order of 1350 C. The pressure in the vacuum coating chamber was maintained at about 25 microns Hg abs. The substrate was moved over the source of tin vapors at a speed of about 150 feet per minute. The tin coating produced was undesirably thin, brown and nonspecular in appearance.

Numerous types of transparent plastics may be employed in. manufacturing the novel thread of the present invention. Examples of such transparent plastic sheets are other polyesters; cellulosic derivatives, such as cellulose acetate and butyrate; polymerized oletins, such as polyethylene and polypropylene; acrylics, vinyl polymers, polyfluoroethylenes obtained from fluorinated ethylenes and copolymers of fluorinated ethylene with unsaturated monomers such as polytetrailuoroethylene and tetrafluoroethylene with chlorotritiuoroethylene.

Since certain changes can be made in the above process and product without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is: l

1l. In the process of producing a vacuum deposited coating of tin on a ilexible nonmetallic substrate the improvement which comprises moving said substrate past a source of tin vapors in a vacuum chamber evacuated to a pressure below about microns Hg abs. while heating molten :tin in said source to a temperature in excess of 1400" C., condensing said high temperature tin vapors on the substrate to form a dense, shiny adherent tin coating on the substrate, exposing said substrate to the tin vapors for a sufficient time -to provide a tin coating of less :than about 5 microinches and oxidizing the tin coating by exposing the freshly-deposited tin coating to condensing steam.

2. In the process of producing a vacuum deposited coating of tin on a plastic substrate, the improvement which comprises moving said substrate past a source of tin vapors in a vacuum chamber evacuated to a pressure below about 100 microns Hg abs. while heating molten tin in said source to a temperature in excess of 1400 C., condensing said high temperature tin vapors on the substrate t-o form a dense, shiny adherent tin coating on the substrate and moving said substrate at ya sufficient rate to provide a thickness of the `tin coating in excess of about .5 microinch, and oxidizing said tin coating to provide scui resistance substantially immediately after deposition of said tin coating by exposing the freshly-deposited tin coating to dry steam.

3. In the process of depositing a coating of tin on a ilexible substrate in a vacuum chamber, the improvement which comprises evacuating the chamber to a pressure below about 100 microns Hg abs., heating molten .tin in a container to a temperature in excess of 1400 C. to provide high temperature tin vapors, condensing said tin vapors lon the substrate to form a dense, shiny adherent tin coating, and lsubjecting said tin coating to heated moist air.

4. In the process of producing a vacuum deposited coating of tin -on a flexible nonmetallic substrate, the improvement which comprises moving said substrate past a source of tin vapors in a vacuum chamber evacuated to a pressure below about 100 microns Hg abs. while heating molten tin in said source to a suiciently high temperature .above 1400 C. to provide an evaporation rate of tin in excess of about?. grams per minute per square inch of molten tin surface, condensing the tin vapors on the substrate to form Ia dense, shiny adherent tin coating on the substrate and moving said substrate at a sufficient rate to provide a thickness of the tin coating of between 1 and 5 microinc-hes, 4and oxidizing said tin coating to provide scufr resistance substantially immediately after deposition of said tin coating by exposing the freshly-deposited tin coating to -hot humid air.

S. -In the process of producing a vacuum deposited coating of tin on a exible paper substrate the improvement which comprises moving said substrate past a source of tin vapors in a vacuum chamber evacuated to a pressure below about 100 microns Hg abs. while heating molten tin in said `source to Va temperature in excess of 1400 C., condensing said high temperature tin vapors on the substrate to forma dense, shiny adherent t-in coating on the substrate and moving said substrate at a sufficient rate `to provide a thickness of the tin coating of between 1 and 5 microinches, and exposing the freshlydeposited :tin coating to condensing steam.

lReferences Cited by the Examiner UNITED STATES PATENTS 6 2,665,223 1/1954 Clough et yal 117-107 2,812,270 11/ 1957 Alexander 117-107 2,975,075 3/1961 Beese 117-107 2,996,410 8/1961: Hnilicka 117-107.1

3,011,919 12/19611 Niklas 117-107 3,013,919 12/1-961 Bialy 154-53.6

3,028,28 1` 4/1-962 Karass 154-536 3,036,933 5/ 1962 Caswell 117-107 OTHER REFERENCES Kubaschewski et al.: Oxidation of Metals and Alloys (1953), Academic Press, New York (pages 208 to 210 relied on).

lPowell et al.: Vapor Plating, John Wiley and Sons (1955), pp. 36-37 relied on.

RICHARD D. NEVIUS, Primary Examiner.

JOSEPH B. SPENCER, Examiner.

R. E. HOWARD, A. GOLIAN, Assistant Examiners. 

1. IN THE PROCESS OF PRODUCING A VACUUM DEPOSITED COATING OF TIN ON A FLEXIBLE NONMETALLIC SUBSTRATE THE IMPROVEMENT WHICH COMPRISES MOVING SAID SUBSTRATE PAST A SOURCE OF TIN VAPORS IN A VACUUM CHAMBER EVACUATED TO A PRESSURE BELOW ABOUT 100 MICRONS HG ABS. WHILE HEATING MOLTEN TIN IN SAID SOURCE TO A TEMPERATURE IN EXCESS OF 1400*C., CONDENSING SAID HIGH TEMPERATURE TIN VAPORS ON THE SUBSTRATE TO FORM A DENSE, SHINY ADHERENT TIN COATING ON THE SUBSTRATE, EXPOSING SAID SUBSTRATE TO THE TIN VAPORS FOR A SUFFICIENT TIME TO PROVIDE A TIN COATING OF LESS THAN ABOUT 5 MICROINCHES AND OXIDIZING THE TIN COATING BY EXPOSING THE FRESHLY-DEPOSITED TIN COATING TO CONDENSING STEAM. 